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The present invention relates to a process of removing hydrogen sulfide from natural gas or other industrial gas, in an integrated system where sulfur is produced.
One of the most common systems for processing gases containing hydrogen sulfide and producing sulfur involves the use of well-known absorber-stripper steps to separate H2S and the well-known Claus process to produce sulfur. In such system, in simplified form, the basic steps are usually:
(a) H2S removal from sour gas, using an H2S absorbent, to obtain sweetened product gas.
(b) Stripping H2S out of the H2S-rich absorbent to obtain H2S.
(c) H2S combustion to obtain SO2 and H2S.
(d) Solid-catalyzed H2S reaction with SO2 at high temperature to form and recover S and to make an off-gas containing reduced amounts of H2S and SO2.
(e) Treating the off-gas from step (d) to recover as S a major fraction of the remaining amounts of H2S and SO2 and to form a stack gas that may be released to the atmosphere.
Steps (c) and (d) in combination are often regarded as the Claus process.
A system that is directed to treating sour gas but does not include reaction of H2S to form sulfur is shown in FIGS. 14-25 of Kohl and Riesenfeld, Gulf Publishing Co., 1985 xe2x80x9cGas Purificationxe2x80x9d, 3rd Edition. FIGS. 14-25 in the Kohl et al. reference shows the basic steps of (a) H2S removal from sour gas using an absorbent to take out the H2S, so as to obtain treated (sweetened gas) of reduced H2S content out the top of the absorber or xe2x80x9ccontactorxe2x80x9d and H2S-rich absorbent out of the bottom of the absorber; and (b) stripping H2S out of the H2S-rich absorbent, by a flash regeneration technique and a heated regeneration technique to strip H2S from the absorbent and obtain H2S and regenerated (lean) absorbent for reuse in step (a).
The system illustrated in the Kohl et al. reference uses a physical absorbent, such as propylene carbonate.
A chemical solvent could be used in that basic-type system, possibly without the flash regeneration part of step (b). Examples of known chemical-type absorbents include amines, such as monoethanolamine (xe2x80x9cMEAxe2x80x9d).
Just as Kohl et al. reference at FIGS. 14-25 is directed to H2S absorption/stripping steps, also FIG. 5 from a paper by Lynn et al., xe2x80x9cThe University of California Berkeley""s Sulfur Recovery Process: Claus Revisitedxe2x80x9d, 1999 Sulfur Recovery Conference, Austin, Tex., Oct. 24-27, 1999, shows the resultant H2S from absorption/stripping can be routed to a reactor. The reactors illustrated in the October 1999 paper are used in combination with a Claus plant (see, for example, the furnace illustrated in FIG. 4). The SO2-rich gas from the furnace is routed to the bottom of an SO2 absorber column. The SO2 is cooled in the bottom of the SO2 absorber using a cooled organic solvent (SO2 absorbent) that is recirculated, through a solvent quench heat exchanger, in a loop at the bottom of the SO2 absorber.
The October 1999 paper also shows in FIG. 4 a process flow diagram for a typical Shell Claus Off-Gas Treatment (SCOT) unit. It is well known in the sulfur recovery industry that a SCOT unit may be used downstream of a Claus plant as a tail-gas clean-up unit (TGCU) to increase the recovery of sulfur from what otherwise would be achieved by only using a conventional Claus plant.
The FIG. 4 illustration of the SCOT unit shows steps including (a) combining a reducing gas with the Claus tail-gas, (b) reducing (hydrogenating) the tail gas containing SO2, S, COS, and CS2 in the SCOT reactor to obtain an H2S-rich stream, (c) quenching the H2S-rich stream by direct contact with water in a quench tower, (d) H2S absorption/stripping steps to produce an H2S stream, and (e) recycle of the H2S stream to the Claus plant. Thus, FIG. 4 of the October 1999 paper is an example of the use of a direct contact aqueous quench in a sulfur recovery process, though to cool an H2S-rich gas, not an SO2-rich gas, as in the present invention.
Another reference which illustrates a process similar to that shown in FIG. 4 from the October 1999 paper, is Naber et al. xe2x80x9cNew Shell Process Treats Claus Off-Gasxe2x80x9d, Chemical Engineering Progress, Vol. 69, No. 12, page 29, December 1973.
The Claus process itself, which consists of a series of reactors in which SO2 and H2S react to form water and sulfur vapor. The reaction is equilibrium-limited at temperatures above the dew point of sulfur vapor. The gas stream leaving each reactor is near chemical equilibrium. In normal operation, most of the sulfur is condensed between reactors to allow further reaction in the next stage. However, in the Claus process, the condensers operate above the dew point of water to avoid forming Wackenroder""s liquid (a dilute aqueous mixture of colloidal sulfur and a solution of sulfoxy acids; see Hackh""s Chemical Dictionary, Fourth Edition, 1969) and the problems that their formation would present. This is done even though the presence of water vapor in the gas stream limits the extent of reaction that can be achieved and thus necessitates the installation of a tail-gas treatment process.
My prior International patent application WO 99/12849, which is hereby incoporated herein by reference, describes a process in which gaseous hydrogen sulfide (H2S) reacts with gaseous sulfur dioxide (SO2) in the presence of an organic liquid or solvent wherein the following reaction occurs:
2H2S(g)+SO2(g)xe2x86x923S(l)+2H2O(g)xe2x80x83xe2x80x83(1)
In the reactor, it is desired to operate above the melting point of sulfur. The reacting gases may flow co-currently or counter-currently to a stream of the organic liquid. A preferred example of such a reactor is a tray-type column in which the reacting gases flow counter-currently to a stream of the organic liquid. The sulfur produced by Reaction (1) in either type of reactor forms a separate liquid phase that flows co-currently with the organic liquid.
The gaseous sulfur dioxide is produced by combustion of hydrogen sulfide. Preferably this combustion is conducted fuel-rich to avoid the risk of forming SO3 and NOx, both of which are undesirable. However, if the combustion is fuel-rich, then elemental sulfur forms in addition to SO2 and will be condensed and partially dissolved in the solvent used in the SO2 absorber, which is undesirable. On the other hand, if the combustion is carried out fuel-lean, the free oxygen that accompanies fuel-lean combustion can cause degradation of the solvent in the SO2 absorber. Furthermore, water vapor is formed by the combustion of H2S and any hydrocarbons that are present in the acid gas fed to the furnace. Most of the water vapor will also condense in the solvent in the SO2 absorber. The presence of water vapor together with the SO2 requires additional cooling, or a higher solvent flow, in the absorber. In addition, that water must be boiled out of the solvent in the SO2 stripper, thereby increasing the energy required in operating the stripper. Furthermore, most of this added water vapor must be condensed from the SO2 leaving the stripper before the latter enters the reactor column to avoid an excessive vapor flow within the reactor column.
According to the present invention, a process is provided for removing H2S from an H2S-rich gas and producing sulfur, which comprises:
(a) reacting H2S in the H2S-rich gas with SO2 to produce sulfur and a reactor off-gas containing H2S and H2O;
(b) combusting the reactor off-gas to produce a combustion gas containing SO2, water vapor;
(c) cooling the combustion gas from step (b) to condense water vapor and sulfur and produce an aqueous stream comprising primarily water; and
(d) introducing the aqueous stream from step (c) into the reactor to provide cooling for the reaction of step (a).
In one aspect, the invention comprises:
in a process for removal of H2S from an H2S-rich gas, in which the H2S-rich gas is reacted with SO2 in a reactor in the presence of an organic liquid to produce sulfur, and in which H2S is combusted to produce a combustion gas containing SO2, water vapor, and in which the SO2 is thereafter reacted with the H2S-rich gas, the steps comprising:
(a) cooling the combustion gas to condense water vapor and sulfur and produce an aqueous stream comprising primarily water; and
(b) introducing said aqueous stream into the reactor to provide cooling for the reaction between the H2S-rich gas and the SO2.
In another aspect, the invention comprises:
a process for removing H2S from an H2S-rich gas and recovering sulfur, which process comprises feeding the H2S-rich gas and an SO2-rich gas, the H2S being in stoichiometric excess, to a reactor column in the presence of a solvent that catalyzes their reaction to form liquid sulfur and water vapor; wherein aqueous streams are injected at one or more points of the reactor column to absorb a part of the heat of reaction by water vaporization; wherein the H2S-rich off-gas is scrubbed with an aqueous stream in the upper section of the reactor column to recover solvent vapor and unreacted SO2 and is then cooled to condense water prior to combusting the H2S-rich off-gas to produce SO2 to be fed to the reactor column; absorbing SO2 from the combustion gas by contacting the gas with an SO2 absorbent in an absorber to obtain an SO2-rich absorbent; and stripping SO2 from the SO2-rich absorbent to obtain an SO2-rich gas; which process further comprises:
(a) burning the cooled H2S-rich gas with an amount of O2-rich gas in a furnace such that substantially all hydrogen is converted to H2O, and at least 90%, preferably about 98% to 99%, of the sulfur is converted to SO2 while at least 0.1%, preferably about 1% to 2%, of the sulfur is converted to S vapor;
(b) cooling the SO2-rich gas from step (a) by direct contact with cooled water in a separate contacting device or water introduced into the lower part of the SO2 absorber to condense H2O and S vapor to produce an aqueous slurry containing suspended sulfur and to obtain cooled SO2-rich gas;
(c) absorbing SO2 from the cooled SO2-rich gas in an SO2 absorber by contacting the gas with an SO2 absorbent to obtain an SO2-rich absorbent and a stack gas of low sulfur compound content; and stripping SO2 from the SO2-rich absorbent to obtain an SO2-rich gas; and
(d) using the aqueous slurry from step (b) as one of the aqueous streams injected at one or more points of the reactor column to absorb a part of the heat of reaction by water vaporization.